As is well-known, an electronic still camera apparatus for converting an optical image of a photographed object into an electrical image signal using a solid-state imaging device, converting the image signal into digital image data, and recording the data in a semiconductor memory, has been developed. In this type of electronic still camera apparatus, a memory card including a semiconductor memory in a card-like case is formed detachably from the body of the camera apparatus so that the memory card can be handled in the same way as a film of a normal camera is.
At present, the memory card of the electronic still camera apparatus is being standardized, and a large memory capacity semiconductor memory for recording a plurality of items of digital image data is required as the semiconductor memory included in the memory card. For example, an SRAM (Static Random Access Memory), a mask ROM, an EEPROM capable of electrically writing and erasing data, and the like are taken into consideration, and a memory card using the SRAM has been already commercialized.
While the memory card using the SRAM has advantages wherein it can be adapted to a data structure of whatever format and the data write/readout speed is high, it has drawbacks wherein a backup battery for holding the written data has to be put into the memory card and thus a memory capacity is reduced by the space for the backup battery and the cost of the SRAM itself is increased uneconomically.
In order to eliminate the drawbacks of the SRAM, the EEPROM has attracted attention as a semiconductor memory used in a memory card. More specifically, the EEPROM attracts attention as a recording medium in place of a magnetic disk, and has the advantages of requiring no backup batteries and decreasing the cost of a chip which are different from those of the SRAM. Therefore, the EEPROM is greatly improving in using as a memory card.
FIG. 1 shows the merits and demerits of a memory card using an SRAM (SRAM card) and a memory card using an EEPROM (EEPROM card). With respect to the backup battery of item 1 and the cost of item 2 in FIG. 1, as described above, the SRAM card has problems wherein it requires a backup battery and increases in cost, and the EEPROM card has advantages wherein it requires no backup batteries and reduced in cost.
The write speed of item 3 and the readout speed of item 4 each can be considered in two different modes one of which is a random access mode common to the SRAM and EEPROM in which data is written/read out to/from an arbitrary byte or bit designated by an address and the other of which is a page mode proper to the EEPROM in which a page having a plurality of successive bytes (several hundreds of bytes) is designated and data is written/read at once for each page.
In the random access mode, the write and read speeds of the SRAM are both high, while those of the EEPROM are both low. Since, in the page mode, the EEPROM writes/reads a large amount of data for one page at once, its data write/read speed is higher than in the random access mode.
The erase mode of item 5 is a mode proper to the EEPROM and not included in the SRAM. More specifically, when the EEPROM writes new data in a region including old data, that is, when the EEPROM rewrites data, it cannot write the new data unless the old data is erased. For this reason, the erase mode is executed when data is written.
The write verify of item 6 is also a mode proper to the EEPROM and not included in the SRAM. Since the EEPROM cannot write data completely by a single normal operation, it is necessary to check whether data is correctly written to the EEPROM by reading the content of the data from the EEPROM every write operation for the EEPROM. This is the write verify.
More specifically, data to be written to the EEPROM is recorded in advance in a buffer memory, and the data is then transferred from the buffer memory to the EEPROM and written thereto. After that, the content of the data written to the EEPROM is read out and compared with that of the data recorded in the buffer memory to determine whether these contents coincide with each other. If it is determined that they do not coincide (error) by the write verify, the content of the buffer memory is repeatedly written to the EEPROM.
As is evident from the above comparison, while the EEPROM has the advantages of requiring no backup batteries, decreasing in cost, and allowing data to be written/read out for each page, it has the disadvantages of decreasing the data write/readout speed in the random access mode and requiring the modes such as the erase mode and write verify that the SRAM does not have.
In the case where the currently-used SRAM is replaced with an EEPROM as a semiconductor memory used in a memory card, it is necessary to make various improvements in the EEPROM so that the problems of decreasing the data write/readout speed, requiring the erase mode and write verify, and the like can be resolved, and the EEPROM can be handled in the same way as a memory card including the SRAM is, in other words, it can be used like an SRAM card.
The problem of this case is that the memory cell of the EEPROM is deteriorated suddenly when the number of times of data rewrite exceeds a fixed value, and a data write error is easy to occur. In other words, since the EEPROM is developed to record program data and intended to rewrite data in the version-up of programs, it cannot be adapted to a number of data rewrite operations.
If, as described above, an EEPROM is used as a semiconductor memory for a memory card used in an electronic still camera apparatus and the like, in place of an SRAM which has been conventionally used, it is reasonable to frequently rewrite data to the EEPROM. Inevitably, a write error is remarkably increased.
Conventionally, it is determined as a write error that data is not written correctly even though the foregoing write verify process is repeated by a predetermined number of times. Even when a write error occurs in part of the EEPROM, the whole memory card including the EEPROM is treated as a defective, which is very inefficient and uneconomical.
As described above, the conventional memory card including the EEPROM has the drawback of making the memory card very inefficient and uneconomical since the whole memory card is treated as a defective when a write error occurs in the EEPROM.
The present invention has been developed in consideration of the above situation and its object is to provide an economical, practical, and excellent memory card in which an EEPROM can be used when a write error occurs in part of the EEPROM and its memory area can be used effectively.